1. Field of the Invention
The present invention generally relates to turbine-driven multi-stage compressors. In another aspect, the invention concerns an improved methodology for starting up a multi-stage compressor driven by a single-shaft gas turbine.
2. Description of the Prior Art
Gas turbines are commonly used to drive large, industrial compressors, such as those employed in the refrigeration cycles of liquefied natural gas (LNG) facilities. Gas turbines used to drive large compressors generally have a single-shaft or a split-shaft configuration. Compressor systems driven by split-shaft gas turbines are typically easier to start-up, but single-shaft gas turbines are available in higher power ratings. Generally, split-shaft gas turbines either are not commercially available or are not economically viable for use in very high load applications, such as for driving the multi-stage compressors of an LNG facility. Therefore, single-shaft gas turbines are usually selected to drive very large multi-stage compressors in industrial applications.
One disadvantage associated with employing a single-shaft gas turbine to drive a large, multi-stage compressor is the requirement for auxiliary power to help start-up the compressor/turbine system. In the past, such auxiliary start-up power has typically been provided by electric motors. These auxiliary motors run at or near full capacity during start-up to help overcome the inertial and aerodynamic forces of the system. After start-up, the auxiliary motor is shut off or scaled back, as the gas turbine takes over primary responsibility for powering the system. Obviously, the requirement for an auxiliary source of rotational power during start-up adds to the overall capital expense of the system.
Another disadvantage of using a single-shaft gas turbine to drive a large, multi-stage compressor is the potential for creating a vacuum in the system upon start-up, which creates a mechanism for air ingress into the system. While manageable, air-contamination of the working fluid is highly undesirable and can present additional operational and/or safety problems.
Thus, a need exists for an improved system and methodology to efficiently start-up large, industrial multi-stage compressors.